Thin-film resistors



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Apri-129, 1969r Filed May 2, 1966 Sheet,

Jock W. Klemmer,

|NvENToR. Bv.

United States Patent O U.S. Cl. 317-101 7 Claims ABSTRACT F THE DISCLOSURE The disclosed thin-film resistors include a basic resistor intercoupled with ya network of selectively intercoupled individual trimming resistors which add or subtract selected increments of resistance to that of the basic resistor. The smallest value trimming resistor provides a predetermined impedance Z1, while each other trimming resistor provides an impedance equal to a different power of two times Z1. A severable electrical conductor is associated with each trimming resistor to selectively effectively electrically connect or not connect the associated resistor into the network. The trimming resistors may be connected in series, with each severable conductor in parallel with a different trimming resistor; or the trimming resistors may be connected in parallel, with each severable conductor in series with a different trimming resistor.

This invention relates to thin-iilm microelectronics and especially to improved thin-film resistors for use in thin-film integrated circuitry and the like. More particularly, the invention relates to methods and means for fabricating thin-film resistors of highly precise values.

In the past, great diiculty has been encountered in economically mass producing thin-hlm resistors within commerically acceptable tolerances. Thin-lm resistors having close tolerances have been created in the laboratory; however, prior design techniques have not resulted in high quality thin-film resistors which may be mass produced. While basic or primary thin-film resistors have been provided with secondary or trimming resistive portions that could be added to basic thin-film resistors to fabricate thin-film resistors of highly precise values, that approach has not produced high quality thin-film resistors susceptible to mass production techniques. Nor has it produced high quality thin-film resistors which may be monitored by automated fabrication techniques.

Accordingly, it is an object of the present invention to provide a method of making high quality thin-film resistors which lends itself readily to mass production techniques.

It is a further object of the present invention to provide improved thin-film resistors of highly precise values.

It is a still further object of the present invention to provide a type of high quality thin-nlm resistor which, when mass produced, results in a high yield of commercially acceptable thin-film resistors.

It is another object of the present invention to provide a type of high quality thin-iilm resistor which is susceptible to automated fabrication techniques which enhances economic manufacturing thereof.

To achieve these and other objects, there is provided, according to one embodiment of the present invention, a thin-lm component comprising a basic -or primary resistor in the form of a thin-film of resistor material disposed on a suitable substrate. Each primary resistor is in series with one or more current-severable conductive areas, hereinafter called fuses. Each fuse is in parallel with a respective secondary or trimming thin-film resistor likewise in the form of a thin-film of resistive material. The resistance values of the secondary thin-lm resistors are related to each other in a binary number sequence.

ICS

The opening of one or more of the fuses make it possible to add 127 increments of resistance to the basic or primary thin-lm resistor in order to provide a thinlm resistor of highly precise value.

Additional objects, advantages, and characteristic features of the present invention will `become readily apparent from the following detailed description of several embodiments of the invention when taken in conjunction with the -accompanying drawings in which:

FIGURE 1 is a perspective view of a thin-film struc- -ture in accordance with the present invention;

FIG. 2 is a plan view of a series of unconnected thinlm resistors arranged in a binary number sequence in accordance with the present invention;

FIG. 3 is a schematic diagram of a basic thin-lm resistor in series with a parallel arrangement of several thin-iilm trimming resistors and fuses in accordance with the present invention;

FIG. 4 is a schematic diagram of a basic thin-film resistor in parallel with a series arrangement of several thin-lm resistors and fuses in accordance with the present invention;

FIG. 5 is a plan view of a basic thin-lm resistor in series with a parallel arrangement of thin-film trimming resistors and fuses in accordance with the present invention;

FIG. 6 is a plan view of a basic thin-film resistor having several thin-hlm trimming resistors connected in series with it, in accordance with the present invention; and

FIG. 7 is a view of a portion of the thin-film trimming resistors of FIG. 6.

Referring no'w to FIG. l, a monolithic structure 10 comprising a thin-iilm resistor, two terminals and a substrate is shown. The monolithic structure 10 may be part of a thin-film network or part of a sheet containing numerous thin-film resistors. The monolithic structure 10 includes a thin-film resistor 11 which may be made of any of a number of current limiting materials, such as, a metal, an alloy or a metal-dielectric combination. A pre'- erred alloy that is commonly utilized is nickel-chromium. The two terminals, the conductive portion of the structure, attached to opposite ends of the thin-film resistor 11 include respective layers of a protective material 13a and b, a highly conductive material 14a and b, and a current limiting material 15a and b, respectively. The substrate 12 may be made of any of several insulative materials, such as, glass or a ceramic material. While the invention will be described with particular reference to materials having resistive impedances, it should be understood that the practice of this invention is not necessarily limited thereto, but may be practiced to equal advantage utilizing oher impedance, such as, capacitive or inductive.

A preferred method of constructing the monolithic structure 10 is by a photoetching process, commonly known as the wet process. In this process, a thin-film of current limiting material, such as, nickel-chromium, is bonded to the entire surface of a substrate. Then a thiniilm of highly conductive material, approximately forty times the thickness of the current limiting material, is bonded to the entire surface of the current limiting material. The next layer bonded to the entire surface of the highly -conductive material is a layer of the protective material, such as, gold, which is approximately twice the thickness of the layer of the current liimting material. As stated earlier, nickel-chrmoium is a preferred alloy that is commonly utilized in constructing thin-film resistors.

p The nickel-chromium is preferred not only for its current limiting properties, but also because of its adhesive properties which ensure proper bonding to the insulative substrate 12. Thus, nickel-chromium is utilized to form both the thin-film resistor 11 and the respective lower layers a and b of the terminals. The respective middle layers 14a and b of the terminals are commonly formed of copper. The respective top layers 13a and b are commonly formed of gold which is known for its ability to 'withstand atmospheric corrosive impurities.

The final step in creating the monolithic structure includes the photoetching of the unwanted gold, copper, and nickel-chromium in order to form the thin-film resistor 11. The electrons that traverse the thin-film resistor 11 will follow the path of least resistanceupon striking either junction 16a or 16b, depending upon the direction of current flow. In the present construction, the electrons will choose a path provided by the copper, thus, the thin-film resistor 11 will end abruptly at the junctions 116a and 16b. The construction of a thin-film structure similar to that of the monolithic structure 10 is not limited to the aforementioned photoetching process, other methods, such as, a masking process, commonly known as the dry process, may be employed.

It is conventional to discuss thin-film resistors in terms of sheet resistance. The sheet resistance of a thin-film material has the resistance measured between two terminals molecularly bonded to the entire sides of the opposite ends of a thin-film resistor. The value of a thinfilm resistor made of a material with a given sheet resistance will be determined by the size of the resistor and is usually expressed in ohms per square. Assuming that the width of the thin-film resistor 11 is equal to the length of the thin-film resistor 11 and the sheet resistance of the material is 100 ohms per square, then the resistance between the terminals lwould measure 100 ohms. If the resistor area of a thin-film resistor constructed of the same ma terial as that of the thin-film resistor 11 is four times as wide as it is long, the value of the resistance of the thiniilm resistor would be ohms. This would be analogous to placing four resistors similar to thin-film resistor 11 in parallel. On the other hand, if the resistor area of a thinlm resistor constructed of the same material as that of the thin-film resistor 11 is four times as long as it is wide, the value of the resistance of the thin-film resistor would be 400 ohms. This would be analogous to placing four resistors similar to the thin-nlm resistor 11 in series. Thus, the resistance of a given thin-film resistor is -determined by the ratio of the length to the width of such resistor.

Referring now to FIG. 2, there is shown a series of seven thin-film resistors, 17-23, inclusive. All seven of these thin-film resistors are assumed to have been constructed of a material having a sheet resistance of 100 ohms per square. The thin-film resistor 17, having two terminals 17a and 17b attached to its opposite ends, has a width which is four times its length. In light of the foregoing discussion, it follows that the resistive impedance or resistance of the thin-film resistor 17 between the attached terminals 17a and 17b would measure 25 ohms. The adjacent thin-film resistor 18 has the same width as that of the thin-film resistor 17, however, its length is twiceI as long as that of the thin-film resistor 17. Thus, the value of the resistive impedance between the terminals attached to the thin-film resistor 18 would be 50 ohms.

The next three thin-film resistors 19, 20 and 21 are the same length as the thinaiilm resistor 18, however, their widths are 1/2, 1A and 1A; the width of the thinlm resistor 18, respectively. Thus, the value of the resistive impedance between the respective terminals attached to the thin-film resistors 19, 20 and 21 would be 100, 200 and 400 ohms, respectively. The last two resistors in the series, the thin-film resistors 22 and 23, are the same width of the thin-film resistor 21, however, their lengths are 2 and 4 times the length of the thinilm resistor 21, respectively. Thus, the value of the resistive impedance between the respective terminals attached to the thin-film resistors 22 and 23 would be 800 and 1,600 ohms, respectively.

It should be apparent at this time that the thin-film resistors 17-23, inclusive, are arranged in a binary number sequence. If these seven thin-nlm resistors are properly connected to a basic or primary thin-iilm resistor, it would be possible to provide 127 increments of resistance to facilitate the trimming of the basic resistor within a prescribed tolerance.

Prior design techniques have included the use of thiniilm trimming resistors in conjunction with a basic thinlm resistor so that a thin-hlm resistor having close tolerances could be created. However, the prior thin-film trimming resistors were consistently the same size, thus, only multiples of a given resistance could be added to or taken away from the resistance of the basic thin-ilm resistor. By utilizing a series of thin-film resistors arranged in a binary number sequence, a relatively large number of increments of resistance are made available by several thin-film trimming resistors. Thus, higher quality thinlm resistors may be created. Furthermore, a greater yield of commercially acceptable thin-film resistors may be realized from a given number of manufactured thin-film resistors.

Referring now to FIG. 3, there is shown a schematic diagram which includes a basic thin-lm resistor, and a series of thin-film trimming resistors, each trimming resistor having a conducting path in parallel with its resistance path. It is noted that the series of secondary or trimming resistors 33-39, inclusive, are arranged in a binary number sequence similar to that of FIG. 2. While the invention will be described with particular. reference to a series of secondary or trimming resistors comprising seven resistors having different values of impedance related to each other in a binary number sequence it should lbe understood that the practice of this invention is not necessarily limited thereto, but may be practiced to equal advantage utilizing other combinations, Such as, ve or nine resistors.

As the circuit is shown, theoretically the resistance between the terminals 31 and 32 is the resistance of the basic resistor 30, i.e., approximately 500 ohms. The fuses `t0-46, inclusive, though illustrated schematically as fuses, are only strips of thin-hlm material having properties of conductivity. In the physical components, the width of the current-severable conductive areas at the locations illustrated as the fuses 40-46, inclusive, is usually more narrow than the width of the thin-film material that connects the fuses in parallel with their respective trimming resistors. This design is illustrated by FIG. 5. The narrowing of the thin-film material at the location of a fuse facilitates the opening of such fuse, however, this design may not be necessary under all circumstances.

A basic thin-film resistor may be created to within a tolerance of *;5% in the laboratory. However, when mass producing thin-film resistors the yield of such resistors is usually quite low. When basic thinilm resistors are mass produced it is usually necesary to produce resistors having tolerances of il0% in order to provide a high yield of acceptable resistors. Therefore, it is necessary to incorporate trimming resistors with the latter type basic thin-film resistor to provide resistors within commercially accepted tolerances of il%.

As stated earlier, the binary number sequence 0f the resistors 33-39, inclusive, provides 127 increments of resistance to facilitate the trimming of the basic resistor 30 to obtain a desired value of resistance. The resistance of basic thin-film resistor having trimming resistors in series with it is usually designed to be somewhat lower than the resistance desired so that trimming resistors may be added to the basic resistor 30 to obtain the desired value of resistance, i.e., 500 ohms. For example, if the basic resistor 30 was measured to read 424 ohms, the fuses 40, 43 and 44 could be opened, then if one were to measure the resistance between the terminals 31 and 32 he would be measuring the resistance of the basic thin-film resistor 30 and the trimming resistors 33, 36 and 37. The resistance measured between the terminals 31 and 32 would now theoretically read 50 ohms. Thus, a thin-film resistor having a 313% tolerance could be mass produced and then trimmed to within 1% of the value of the desired resistance.

Numerous means may be utilized to open a fuse which is parallel to a trimming resistor, for example, electrically burning, manually handscribing or Sandblasting the fuse to add a resistor. A preferred method is the electrically burning of the fuse to add a trimming resistance to the basic resistor. The electrically burning method is more susceptible to mass producing thin-.film resistors because exterior monitoring equipment may be utilized to check the resistance of the basic resistor, add the necessary trimming resistors to the basic resistor, and the monitor the nal resistance. By automating the manufacture of high quality thin-lm resistors, the present cost of such resistors can be greatly reduced.

When electricallyl burning the fuse, care must be taken in order that the resistor is not harmed. The applicant has utilized a method whereby an electrical pulse having a predetermined level of current is applied to the fuses for a controlled period of time. Thus, the fuse is opened, the resistor is not harmed, and the resistor is now added to the basic resistor.

Referring now to FIG. 4, there is shown a schematic diagram which includes a basic thin-film resistor, and a number of thin-film trimming resistors in parallel with the basic thin-film resistor, each trimming resistor having a fuse which is in series with a respective trimming resistor and also being in parallel with the basic thin-film resistor. The parallel arrangement of the trimming resistors 5359, inclusive, are arranged in a binary number sequence, i.e., the values of resistance decrease from 10,000 ohms to 156.25 ohms, respectively. As the circuit is shown, theoretically the value of resistance between the terminals S1 and 52 is approximately 20 ohms. As in the schematic arrangement of FIG. 3, the fuse 60-66, inclusive, though illustrated schematically as fuses, are only strips of thin-lm material having properties of conductivity. Also, as in the schematic arrangement of FIG. 3, the binary number sequence of the resistors 53-59, inclusive, provides 127 increments of resistance to facilitate the trimming of the basic resistor 50 to within a pre scribed tolerance.

If a basic thin-film resistor has a value of 200 ohms or less, the trimming resistors would necessarily occupy a high percentage of the area of the substrate; therefore, when creating basic thin-f1lm resistors having a low resistance value, the trimming resistors having a higher value of resistance are connected in parallel to the basic thin-film resistor. Thus, a greater packaging density of a given substrate may be achieved. The resistance of the basic thin-hlm resistors having trimming resistors in parallel with it is usually designed to be higher than the resistance desired. For example, if it is desired to have the Value of the resistance between the terminals 51 and 52 be 20 ohms, then the basic resistor y50 would be designed to have a value of resistance of approximately 22 ohms, il0%. Thus, if the basis resistor 50 was just within its upper tolerance and had a value of approximately 24 ohms, the connected trimming resistors would adjust the value of the resistance between the terminals 51 and 52 to below 20 ohms. If the value of the resistance between the terminals 51 and 52 with all the trimming resistors included between those terminals is less than 20 ohms, then one or more of the fuses 60-66, could be opened, and one or more of the respective thin-film trimming resistors could be uncoupled from the circuit, thus the resistance would be increased to the desired value, i.e., 20 ohms.

As in the series arrangement of FIG. 3, the fuses of the parallel arrangement of FIG. 4 can be opened by electrically burning, manually handscribi-ng or Sandblasting the fuses, however, in the latter case a resistance is now eliminated from the resistive component. One advantage of the parallel arrangement of trimming resistors is that no trimming resistors can be harmed when electrically opening the fuses.

Referring now to FIG. 5, a plan view of a thin-film structure 70, including a thin-film resistor, conducting strips, and terminals which have been created on a substrate 71 by the aforementioned wet process as described earlier, is shown. The substrate 71 may consist of any of a number of insulative materials, for example, glass. The electrical components of the thin-film structure 70 illustrate the physical layout of the schematic diagram of FIG. 3.

A basic thin-iilm resistor 73 consisting of a strip of current limiting material, such as, nickel-chromium, is shown between the terminals 72 and 74. A series of seven thin-film trimming resistors y80416, inclusive, are connected in series between the terminals 74 and 75 as shown earlier by the schematic illustrated in FIG. 3. The fuses 90-96, inclusive, are connected in parallel with respective thin-film trimming resistors -86, inclusive.

After the thin-film structure, or thin-lm component, has been created, the resistance of the basic thin-lm resistor 73 should be less than 500 ohms. The value of the resistance of the basic resistor 73 is usually designed to be somewhat lower than the desired value of resistance so that one or more of the 127 increments of resistance made available by the binary number arrangement of thin-hlm trimming resistors 80-86, inclusive, may be added to the basic thin-film resistor to obtain the desired resistance. One or more of the trimming resistors 80-86 inclusive, may be added to the resistance between the terminals 72 and 75 by opening the respective fuses, -96, inclusive.

The construction of a high quality thin-film resistor of the type described above may be fully automated. In developing such a resistor a computer and associated circuitry could be utilized to measure the resistance of the basic thin-film resistor 73. Then the appropriate fuses are opened so that the proper resistance is added to thev basic resistor to obtain the desired resistance. Finally the total resistance between the terminals 72 and 75 is measured, then, the thin-film resistor 70 may be accepted or rejected depending upon whether the value of the resistance is within the proper tolerances.

Furthermore, the thin-film structure 70 may then be inserted into an electronic circuit, not shown. If the character of the electronic circuit necessitates increasing the resistance provided by the basic thin-film resistor 73 and 'the trimming resistors which have coupled to the basic resistor, then one or more additional fuses may be opened to couple one or more respective trimming resistors to the basic resistor.

`Referring now to FIG. 6, another embodiment of this invention is shown. A thin-film structure includes an insulative substrate 101, a basic resistor 111, nine terminals 1024110, inclusive, and seven trimming resistors T12-118, inclusive. The thin-lm structure 100 has been created by the forementioned wet process as described earlier. The substrate 101 may be made from any of a number of insulative materials, for example, glass. The basic thin-film resistor 111 and the `seven thin-film trimming resistors 112-118, inclusive, may be made from several current limiting materials, such as, nickel-chromium. The terminals 102-110, inclusive, may consist of the materials as described in FIG. 1.

`If the desired value of the resistance of the thin-film structure 100 is 500 ohms, then the resistance of the basic thin-hlm resistor 111 should be designed to 'be less than 500 ohms. For example, if the actual resistance of the basic thin-film resistor 111 is 485 ohms, when inserting the thin-film structure 100 within a circuit respect-ive leads may be connected to the terminals 102 and 107 to obtain the desired value of resistance, i.e., 500 ohms.

Referring now to FIG. 7, another embodiment of this invention is shown. A portion of the thin-film structure 100, including the thin-film trimming resistors 112-114,

inclusive, and the terminals S-106, inclusive is shown. A shunting bar 120 is shown connecting the terminals 104 and 105. The shunting bar 120 may be made from any of la number of highly conductive materials, such as, silver, The purpose of the shunting bar 120 and 'any additional shunting bars that may be utilized is to shunt out one or more of the thin-film trimming resistors illustrated in FIG. 6 so that 127 increments of resistance may ,be available to 'be added to the basic thin-film resistor 1`11. One of the advantages of this particular embodiment is that the values of the trimming resistors may be measured prior to their being shunted out o-f the circuit. Thus, one would more likely be assured that the addition of trimming resistors to the basic resistor would result in obtaining a thinlm resistor within a prescribed -toler-a-nce than in the aforementioned method of opening fuses to add a trimming resistor to the basic resistor. However, the aforementioned method of opening the fuses is more susceptible to automation than the method of add-ing s-hunting bars to shunt out resistance from the thin-film structure.

Thus, although the present invention has been shown and described with reference to particular embodiments, nevertheless, various changes and modifications obvious to a person skilled in the art to which the yinvention pertains are deemed to lie within the spirit, scope, and contemplation of the invention as set forth in the appended claims.

What is claimed is:

1. An electrical component comprising:

a supporting member of electrically insulating material;

a number n of intercoupled individual impedance elements formed on said member, where n is a positive integer not less than two, a iirst one of `said impedance elements providing a predetermined im- :pedance value Z1, each other of said impedance elements providing a different impedance value Zk given by 'the relation Zk=2 k 1)Z1, where k is a positive 4integer not less than two and not greater than n identifying the impedance element in question; and

a number n of severable electrically conductive means, each associated with a different one of said impedance elements, for effectively electrically coupling selected ones of said impedance elements into a desired current iiow arrangement.

l2. An electrical component according to claim 1 wherein each of said impedance elements is Va thin-film resistor bonded to said supporting member, and each said electrically conductive means includes a strip of electrical-ly conductive material formed on said supporting member and in electrical contact with the Iassociated one of said impedance elements, each sai-d strip having a notched portion to facilitate severing thereof.

3. An electrical component comprising:

a supporting member of electrically insulating material;

.a number n of individual .impedance elements formed on said member and connected in series between iirst and second terminals, where n is a positive integer not less than two, a first one of said impedance elements providing a predetermined impedance value Z1, each other of said impedance elements providing a -diiierent impedance value Zk given by the relation Zk=(k-1) Z1, where k is a positive integer not less 'than 'two land not greater than n` identifying the impedance element in question; and

a number n of severable electrically conductive means, each connected directly across a diffe-rent one of said impedance elements, for selectively eiectively electrically connecting or not connecting the associated impedance element into a desired current ow path between said lirst and second terminals.

4. An electrical component `according to claim l3 wherein an additional impedance element is connected in series with said individual impedance elements, .said additional impedance element providing an impedance value substantially greater than lthat of any of .said individual impedance elements, whereby said individual impedance elements add selected increments -of impedance to the impedance of said additional impedance element to produce a desired net impedance value.

5. An electrical component comprising: a supporting member of electrically insulating material; a number n of individual impedance elements Iformed on said member and adapted to be connected in parallel 'between iirst .and second terminals, where n is a positive integer not less than two, a iirst one of said impedance elements providing a predetermined impedance value Z1, each other of said impedance elements providing a different impedance value Zk given by the relation Zk=2(k"1)Z1, where k is a positive integer not less than two and not greater than n identifying the impedance element in question; and :a number n of severable electrically conductive means, each connected in series lwith Ia different one of said impedance elements, for selectively elctrically connecting or not connecting the associated impedane element between said first and second terminals. 6. An electrical component according lto claim 5 4wherein .an `additional impedance element is connected between said iirst and second terminals in parallel with connected ones -of said individual impedance elements, said additional impedance element providing .an impedance value substantially less than that of any of said individual impedance elements, whereby said individual impedance elements sufbstract selected increments of impedance from the impedance 'of said additional impedance element to produce a desired net impedance value between said rst and second terminals.

J7. An electrical component comprising:

a supporting member of electrically insulating material;

a number n of individual impedance elements Aformed on said member and connected in se-ries between first land second terminals, where n is a positive integer not l-less than two, a first lone of said impedance elements providing .a predetermined impedance value Z1, each other of said impedance elements providing a different impedance value Zk given by the relation Zk=2 k1r Z1, where k is a positive integer not less than two and not greater than n identifying the -impedance element in question; and

a number n of electrically conductive means, each adapted to be connected directly across a different one of said impedance elements, for effectively electricaly shortcircuiting the impedance element across which it is connected.

References Cited UNITED STATES PATENTS 1,665,499 4/ 1928 Hoch 317-242 2,758,256 8/1956 Eisler 338-314 XR 3,071,749 1/1963 Starr 1", 338-314 3,324,362 6/ 1967 Tassara.

3,361,936 l/ 1968 Umantseu.

LEWIS H. MYERS, Primary Examiner.

I. R. SCOTT, Assistant Examiner.

U.S. Cl. X.R. 3 3 8 3 08 U.S. DEPARTMENT 0F COMMERCE PATENT OFFICE Washington,D.C. 20231 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 ,441 ,804 April 29 196? Jack W. Klemmer It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column Z line l "make" should read makes line 64 "limting" should read limiting line 65 "chrmo ium should read Chromium line 72, "llSa" should read 15a Column 3, line 14, "lla" should read 16a Column 4 line 74 "50" should read 500 Column 5 line l2 "the" after "and" should read then line S7 "basis should read basic Column 6 line 2O after "structure" insert 70 Column 8 line 25 "impedane" should read impedance Signed and sealed this 3rd day of February 1970 (SEAL) Attest:

Edward M. Fletcher, Jr. WILLIAM E. SCHUYLER, JR.

Attesting Officer Commissioner of Patents 

